A more thorough explanation:

A supplemental complaint is an additional complaint filed in a civil action that either corrects a defect in the original complaint or adds relevant matters that occurred after the action began. It is different from an amended complaint, which modifies and replaces the original complaint by adding relevant matters that occurred before or at the time the action began.

For example, if a plaintiff filed a complaint against a defendant for breach of contract, but later discovered additional facts that support their claim, they may file a supplemental complaint to include those facts. Similarly, if a plaintiff filed a complaint but made a mistake in the original complaint, they may file a supplemental complaint to correct the mistake.

A party must obtain the court's permission to file a supplemental complaint, and it is generally used to provide the court with new information that was not available at the time of the original complaint.